CIRCUIT DESCRIPTION

UHF RECEIVER DESCRIPTION

CR202. The function of this filter is to attenuate frequencies outside the receive band such as the first injection, image, and half IF frequencies. The pass- band frequency of the filter is shifted in four steps using PIN diodes. These diodes are controlled by microcontroller through the Q2 and Q3 outputs of shift register U800. The control signals for each of four band segments are listed in Section 4.4.

The PIN diodes present a very low impedance at RF frequencies when forward biased and a very high impedance when reverse biased. This allows them to be used to switch capacitance in and out of the filter. For example, when the lowest segment of the frequency band is selected, both control signals are high and the diodes are forward biased by current flowing through R201-R204. Therefore, C207, C208, C218, and C219 are effectively connected to ground through CR201 and CR202 which lowers the passband frequency of the filter.

Ceramic resonators L200 and L201 have a very high Q and therefore cause very little receive signal loss. Capacitors on the input and output of the filter provide impedance matching with the adjoining stages.

3.8.2 RF AMPLIFIER (Q201)

RF amplifier Q201 improves and stabilizes receiver sensitivity and also recovers filter losses. Several capacitors on the input and also L202 provide impedance matching. CR203 protects the base-emitter junction of Q201 from damage caused by high level input signals.

The bias current of Q201 is fixed at a constant level by Q200. The collector current of Q201 flows through R207. The voltage drop across that resistor (and therefore the current) is set by R205 and R206. For example, if current through R207 attempts to increase, the emitter voltage of Q200 decreases. Q200 then conducts less and turns Q201 off slightly to maintain a constant bias current. This provides a stable bias over changes in temperature.

The output signal of Q201 is fed to another two- pole bandpass filter similar to the one on the input of Q201 as described in the preceding section. Imped-

ance matching with the filter is provided by L203, C227, C228, C234, and C235. Resistor R209 lowers the Q of L203 to make it less frequency selective. C222-C226 decouple various unwanted AC signals from the circuit.

3.8.3FIRST MIXER (Q202), INJECTION AMPLIFIER (Q204)

Q202 is a dual-gate MOSFET mixer. Impedance matching at one gate is provided by C245, R214, and L207. The first injection frequency from the synthesizer is applied to the other gate. Since the first IF is 45 MHz and low-side injection is used, the injection frequency is 45 MHz below the receive frequency.

The signal from the synthesizer is amplified by Q204. A 3 dB pad on the output, consisting of R225- R227, sets the input level to the mixer. A low-pass filter network formed by C262-C264 and L211 attenuates spurious frequencies occurring above the injection frequency band. Q203 provides a stable bias current similar to Q200 described in Section 3.8.2. Temperature compensation is provided by CR206 which mirrors the voltage drop across the base-emitter junction of Q204.

Impedance matching on the output of mixer Q202 is provided at 45 MHz by L208, C251, and C252. The signal is then fed to Z204 which is a four-pole crystal filter with a nominal –3 dB bandwidth of 15 kHz. This filter attenuates wideband noise, adjacent channels, frequencies resulting from intermodulation, and other undesired frequencies. Impedance matching on the input is provided by C251, C252, C266, C268, and L213; impedance matching on the output is provided by C270, C271, C272, L215, and R228.

3.8.4 SECOND MIXER/DETECTOR (U201)

Second Mixer

U201 contains second mixer, IF amplifier, detector, RSSI, and audio amplifier stages as shown in Figure 3-6.The 45 MHz IF signal is applied to pin 2 which is the input of an internal IF amplifier stage. From the IF amplifier the signal is internally fed to the mixer which combines it with the 44.550 MHz second injection frequency to produce a second IF of 450 kHz.

 

February 2001

3-17

Part No. 001-9800-001